U.S. Pat. No. 4,055,705 to Stecura and Liebert discloses a thermal barrier coating system wherein a stabilized zirconia thermal barrier coating covers a NiCrAlY bond coating. Certain problems have been encountered when this system is used in an environment of contaminated fuels.
Tucker, Jr. U.S. Pat. No. 3,837,894 and Weatherly et al U.S. Pat. No. 4,095,003 disclose duplex coatings. Both patents are concerned with the porosity of prior art coatings. In the Tucker, Jr. patent the undercoat is metallurgically sealed by heat treating. Weatherly et al refers to the Tucker patent and points out that not all substrates can be heat treated without degrading the properties of the substrate. The Weatherly et al patent utilizes a bond coating that is formed in two layers and then heat treats prior to the application of the ceramic layer. Takabatake et al U.S. Pat. No. 3,927,223 is directed to refractory based oxide coated membrane on a substrate.
While satisfactory results for a limited time have been achieved with the coatings of the prior art, none are capable of withstanding the exposure of contaminated fuels for the time required for gas turbines and other heat engines.